Movable control surfaces of an aircraft, such as a rudder, are generally hingably mounted to a fixed section, for example a rudder is hingably mounted to a vertical fin of a vertical tail plane. Such a movable control surface typically deflects about a hinge axis extending along the fixed section. The control surfaces generally extend from a trailing edge of a fixed surface, such as a wing or stabiliser. A conventional control surface is deflectable about their mounting.
Boundary layer separation is a phenomenon shown in FIG. 3 in which the boundary layer peels away from a solid surface 26 of an aircraft aerofoil as the result of an adverse pressure gradient opposing the flow along it. When the boundary layer separates, as shown by dashed lines in FIG. 3, the lifting efficiency of the surface drops. For example, if when the angle of the wing increases relative to the airflow, significant areas of separation occurs, the drag increases dramatically, and the aircraft is said to stall.
Boundary layer separation control is therefore very important for aircraft. One consequence of a high deflection angle of a control surface about its deflection axis is that a section of the nose of the control surface becomes exposed at the boundary of the control surface and the fixed section from which the control surface extends. Due to the curvature of the control surface nose becoming exposed to the air flow, significant boundary layer separation may occur.
If the flow separation at high angles of deflection of the control surface could be controlled, then the performance of the aircraft would be improved.